Step 1: First you need a large barrel potato gun.

This is the threaded end, where the fuel goes (I use ether). I added screws in addition to the ABS glue for extra strength, although it probably isn't necessary. It is also your access to the ignitor (see next step) and how you ventilate the cannon between shots, to get some fresh air inside quickly.

Step 2: The ignitor.

I put mine about 8 inches from the end cap. Close enough to work on. It is a replacement barbeque ignitor that comes with a AAA battery powered ignition switch. I sealed the holes with silicone. You can bend the wire electrode until you get the gap that's right for you. On the outside, I reinforced and protected the connections with epoxy, since they are a little fragile for this application.

Step 3: The firing button.

If you are unfamiliar with barbeque ignitors, the battery is underneath the red button. I put mine too far along the barrel to use fasteners which might protrude into the barrel. So it's two-part epoxy and lots of black tape. I stretched out the supplied wires as far as they would go.

Step 4: The most important part.

Once you have built a 4 inch barrel potato gun, you will probably realize that pototoes are too small to touch the sides and you can't build any compression. Rutabegas work, but firing off 2 dozen of those gets expensive. Wads of wet newspaper is labor-intensive and messy. Bigger suddenly isn't better. However, if you take a yogurt or cottage cheese container, and trace around it snugly in the barrel, you get a perfect fit.

When you are done, cut off the excess, and see if it fits tightly.

Now, for a little strength and better compression, wrap the surface that will contact the barrel with some electrical tape.

a potato gun is a big cannon, about the same size as this cannon, but it launches potatos hundreds of feet. It is powered by a chamber full of hairspray. You stick the potato in, light the hairspray, and watch the potato burst through walls. I mad a mini one out of a chapstick container once. It was cool!

hairspray is for whimps! you need to go pneumatic if you want to go "hundreds of feet". A pneumatic potato cannon has an air chamber filled to 80-90 psi, and a sprinkler, ball, butterfly, or piston valve right after the air chamber. when you open the valve, the air pushes the projectile out the barrel.

I've built a similar device and found that you can nearly double the range/power by putting the container in the other way (opening facing down.) This way the air pressure will expand it, making a better seal, rather than compressing it and creating leaks around the edges.

For the record, I launched lollipops out of a model rocket. Yeah, I got hit in the head with one falling from 500 feet. No, I did not crack my "scull". It's friggin CANDY. There's very little momentum and it's mostly prone to shatter if it hits something firm.

Nah, the candy will break before anything bad happens. The momentum will be about the same from 15,000 feet as it will from 150 feet. Candy is not very aerodynamic (Well, I suppose Tootsie Pops are pretty bullet-like) and it will have a slow terminal velocity. Combined with the small mass the momentum will be less than knock-someone-out strength. Moral: No Tootsie Pops. :P

I had encountered the wadding problem with my cannons as well. I found that a simple paper cup works if you mark the outside of the cup where it becomes snug in the barrel and then cut down to that point from the rim at the 12 3 6 and 9 o'clock points of the rim. Basically it functions much like a shotgun wadding and lands a few feet away.